Chemical vapor deposition (CVD) involves the formation of a solid film on a substrate by the reaction of vapor phase chemicals that contain the required constituents. The reactant gases are introduced into a reactant chamber and are decomposed and reacted at a heated surface to form the film. CVD techniques are widely employed in the fabrication of semiconductor structures; therefore, CVD techniques are some of the fundamental building blocks in semiconductor processing. Low pressure chemical vapor deposition (LPCVD) techniques involve, as the name implies, low pressures and are advantageous in that films having high purity, excellent uniformity, and conformal step coverage are obtainable. LPCVD techniques are disadvantageous in that high temperatures are typically required.
Another disadvantage associated with LPCVD processes involves the formation of tetraethylorthosilicate (TEOS) or Si(OC.sub.2 H.sub.5).sub.4 films (or silicon dioxide layers made from TEOS starting material). During the LPCVD TEOS film formation process, the starting gas is continuously introduced into the reactor vessel, but a vacuum is continuously drawn from the reactor vessel through a vacuum extraction system connected to thereto, so that a predetermined low pressure is maintained in the reactor vessel to properly control the growth of the TEOS film. In many instances, the starting gas is not completely used in the formation of the TEOS film, and a large part thereof is undesirably pulled from the reactor vessel into the vacuum extraction system. As a result, the pump/vacuum system responsible for the low pressures, such as a mechanical booster pump, a rotary pump, a blower, and other related pump system elements, are damaged by the deposition therein of the starting gas pulled from the reactor vessel. In particular, the starting gas pulled from the reactor vessel is undesirably deposited in the pumps thereby creating an unwanted build-up in an interior of the vacuum extraction system. TEOS byproducts also create an unwanted build-up in an interior of the vacuum extraction system. TEOS material build-up is characterized by a very sticky film. This is problematic because the build-up on internal surfaces of the LPCVD apparatus and particularly on internal movable parts of the pumps requires frequent vigorous cleanings.
Moreover, TEOS is particularly problematic in LPCVD processes because TEOS gas condenses in the vicinity of 35.degree. C. Problems ensue when an LPCVD TEOS film formation process is temporarily stopped, such as during evening hours or during a power glitch causing the pump to shut off. In particular, after the LPCVD apparatus is turned off, the apparatus cools down to room temperatures from its high operating temperatures. Since TEOS gas condenses around 35.degree. C., TEOS materials build-up on various elements of the LPCVD apparatus including the pump/vacuum system as the temperature of the LPCVD apparatus cools down. Upon turning the LPCVD apparatus back on, blower vains, rotors and other elements of the pump/vacuum system cannot function properly due to the solid build-up of TEOS material. In many instances, within only 30 minutes after stopping and LPCVD TEOS film formation process, TEOS material builds-up so extensively that it is impossible to turn the LPCVD apparatus back on. Although it is sometimes desirable to temporarily interrupt the LPCVD TEOS film formation process in order to make minor process adjustments, such interruptions are avoided due to the possibility that the LPCVD apparatus cannot be turned back on.
Cleaning a pump/vacuum system in an LPCVD TEOS film formation process requires one to disconnect the pump/vacuum system from the LPCVD apparatus, disassemble the pump/vacuum system, clean the individual parts, reassemble the pump/vacuum system, connect the pump/vacuum system back to the LPCVD apparatus, and testing the cleaned system. This cleaning process typically takes three to four days, which is a long period of time to be off-line. Not only is this cleaning process cumbersome, but the LPCVD apparatus is rendered useless during the long cleaning time, thus inhibiting further semiconductor processing.